Pandora on my Roku 3 sounds great. Roku-->HDMI-->TV-->digital coax-->AVR-->3.1
The only downsides are (1) running my TV for Pandora, hooking something up to the AVR and then casting Pandora would make more sense. If I cared enough, and (2) the Pandora app sucks, I can't add tracks to a station or use a lot of other features. I guess it's good in a pinch.
I'm fine with Pandora on Roku. Spotify, on the other hand, stinks on Roku. They haven't updated the Roku channel is more than two years.
2017 Roku Ultra, 2017 Roku Streaming Stick+, 2nd Gen. Roku 3
Apple TV 3, 1st Gen. Fire TV Stick
1 Mohu Sky, 1 Mohu Leaf Ultimate antenna
First Generation Dual Tuner Tablo
HP Stream Mini (Plex/PlayOn/iTunes/Web)
(Roku owner since 2011)